"wheezy-updates" contains important updates for functionality of packages
(e.g. new signatures for virus scanners), but no security updates.
"wheezy-proposed-updates" provides preliminary updates for inclusion in
the next point release of wheezy (security updates and more).

"squeeze-updates" contains important updates for functionality of packages
(e.g. new signatures for virus scanners, but no security updates), called "volatile" in previous releases.
"squeeze-proposed-updates" provides preliminary updates for inclusion in
the next point release of squeeze (security updates and more).